Automated-guided vehicles are usually used in severe environments where a worker has a difficulty or an inefficiency in direct performance of a task. That is, the AGVs transfer articles or carry out a predetermined task by a manipulator through an automatic control, while stopped at work stations provided along a predetermined travelling path.
FIG. 1 is a perspective view of a conventional AGV, and FIG. 2 is a top plan view of the AGV located on a travelling path, for illustrating a conventional method for correcting a stop position of the AGV.
The AGV is formed with a vehicle body 51 and a manipulator 52 mounted on the top of the vehicle body 51 for carrying out a predetermined task. Under both sides of the vehicle body 51 a pair of drive wheels 53 are installed for driving the AGV. Wheel driving motors 55 for driving the drive wheels 53 are installed beside the drive wheels 53. In the front and rear ends of the vehicle body 51 a pair of track sensors 57 are installed for detecting electromagnetic waves from a travelling direction mark (not shown) on the travelling path 70. A controller (not shown) controls the wheel driving motors 55 and determines the travelling direction of the AGV, according to the values of the electromagnetic waves detected by the track sensors 57.
On the travelling path 70 a stop mark 75 made of magnetic material is attached at the point or workplace at which the AGV has to stop. On the bottom of the vehicle body 51 a stop sensor 60 is installed for sensing a magnetic field from the stop mark 75. On one lateral side of the vehicle body 51 first and second sensor groups 62 and 64 are installed along the longitudinal direction of the vehicle body 51. Each of the first and second sensor groups 62 is comprised of a plurality of ultrasonic wave sensors. On a side wall 71 installed beside the travelling path 70 of the AGV, first and second reflectors 72 and 74 are installed for reflecting ultrasonic waves from the sensor groups 62 and 64. The first and second reflectors 72 and 74 are installed with the same interval as that of the first and second sensor groups 62 and 64 and at the same height as that of these sensor groups 62 and 64, to correspond to each other.
With this configuration, when the AGV stops according to the signal that the stop mark 75 is detected from the stop sensor 60, ultrasonic waves are (a) generated from the first and second sensor groups 62 and 64, (b) reflected by the first and second reflectors 72 and 74, and (c) then returned to the first and second sensor groups 62 and 64. The controller determines an actual stop position of the AGV based on the strength of the returned ultrasonic waves.
More particularly, coordinates are set initially wherein the center of the vehicle body 51 is the origin (0), the travelling path 70 is a Y-axis, and the perpendicular axis to the Y-axis and passing through the origin is an X-axis. Then, the distance a between the first sensor group 62 and the first reflector 72 on the X-axis, and the distance b between the second sensor group 64 and the second reflector 74 on the X-axis, are calculated. These distances a and b can be obtained by multiplying the speed of the ultrasonic waves by the time consumed in returning to the sensor groups. The angle .theta. by which the vehicle body 51 deviates from the travelling path 70 is obtained by using the difference a-b between a and b and the distance d between the first and second sensor groups 62 and 64.
To determine how far the vehicle body 51 is deviated from the stop mark 75 in the travelling direction thereof, a distance c between the sensor group 62 or 64 and the reflector 72 or 74 on the Y-axis is calculated. The distance c can be calculated by using the strength of the ultrasonic waves generated by the sensor group 62 or 64 and returned thereto after reflecting from the reflector 72 or 74.
According to the actual stop position of the vehicle body 51 of the AGV determined as described above, the controller controls the wheel driving motors 55 and relocates the vehicle body 51 at a predetermined correct working position.
However, in the conventional AGV position correcting method, a multiplicity of ultrasonic wave sensors are required, thereby raising the cost of production. Further, since the reflectors 72 and 74 have to be installed corresponding to the ultrasonic wave sensor groups 62 and 64 of the AGV, the work of installation is complicated.
In the conventional art, after relocation of the vehicle body 51 is conducted according to the value calculated from the comparison of the actual stop position of the vehicle body 51 and the predetermined correct stop position of the vehicle body, the manipulator carries out the predetermined work. Therefore, the AGV has to be inactive until the relocation of the vehicle body 51 is finished, thereby increasing an overall time for completing the task.
With the conventional art using the ultrasonic waves sensors, only a relative position of the vehicle body 51 to the travelling path is determined. Therefore, an extra device is needed to determine a reference position by which a correct working position of the vehicle body on the travelling path is taught, thereby increasing the cost of production.